The following discussion will consider (a) reasons why it is desirable to change pitch in an aircraft propeller and (b) two forces which oppose attempts to change pitch.
The two forces are the centrifugal twisting moment and the gas twisting moment of the propeller blades. These forces, if acting unchecked, tend to drive the pitch to an improper position, and so the pitch change mechanism must overcome them. In case the pitch-change mechanism fails, a back-up system is generally provided to overcome these two forces.